Polyimide-containing coating composition and film formed from the same

ABSTRACT

To provide a coating composition of a soluble aromatic polyimide having good heat resistance and mechanical strength. As a soluble polyimide, a compound of the formula (I) in which R&#39;s represent independently from one another H, halogen, an alkyl group or an alkoxy group, or a substituted or unsubstituted phenyl group; A is a tetravalent aromatic group or a group of the formula (II) in which B is a covalent bond, a &gt;C(R 2 ) 2  group, a carbonyl group, O, S, a &gt;SO 2 , group, a &gt;Si(CH 3 ) 2  group, a &gt;Si(C 2 H 5 ) 2  group, a &gt;NR 3  group or a bifunctional ether group in which R 2  is a hydrogen atom or —C(R 4 ) 3 , R 3  is H, an alkyl group or an aryl group, and R 4  is H, F or Cl; Ar is an aromatic nucleus or a group of the formula (III) in which Ar and B are the same as defined above, and each p is an integer of 0 to 10; m is an integer of 1 to 10 is used. The polyimide has a weight average molecular weight of 150,000 to 1,000,000.

FIELD OF THE INVENTION

The present invention relates to a coating composition comprising apolyimide soluble in organic solvents (soluble polyamide) and a filmformed therefrom.

Polyimide is known as an egineering plastic, which has substantiallyconstant heat resistance, electric insulation, mechanical propertiessuch as flexibility, etc., and radiation resistance in a relatively widetemperature range from a very low temperature to a high temperature(e.g. 250 to 300° C.), and is used in aerospace industries, electric andelectronic industries which lay severe conditions to materials.

The polyimide has imide bonds in the backbone, and usually prepared bythe polycondensation of an acid anhydride and a diamine. In particular,an aromatic polyimide comprising an aromatic acid anhydride and anaromatic diamine has good heat resistance.

Aromatic polyimide is supplied in the form of a film or a moldedarticle. Usually, the film and the molded article are not directlyproduced from the aromatic polyimide, since many aromatic polyamides donot have a clear melting point or there is no solvent suitable fordissolving the aromatic polyamides as such.

In general, a polyamic acid, which is a precursor of an aromaticpolyamide is dissolved in a solvent to form a solution, and then thesolution is processed to form a film and so on, as disclosed inJP-A-10-96826 and JP-A-10-171265. The solution of the precusor polyamicacid is stored and treated under dry conditions, since the polyamic acidis easily decomposed and has low storage stability. The processing ofthe polyamic acid solution involves heating to remove the solvent andalso to convert the precursor to the aromatic polyimide through adehydration ring-closing reaction (imidation reaction). In the imidationreaction, water is formed as a by-product. Therefore, when the drying iseffected quickly, the traces of bubbles remain in the polyimide film.Thus, the above heating is often carried out over a long time.

Also, aromatic polyamides which can be soluble in the solvents after thecompletion of imidation (soluble aromatic polyimides) are known. Thesoluble aromaci polyimides are prepared usingdiphenylsulfone-3,3′,4,4′-tetracarboxylic anhydride as an acidanhydride, and commercially available in the name of RIKACOAT fromSHIN-NIPPON RIKA KABUSHIKIKAISHA.

The soluble aromatic polyimides can form an aromatic polyimide filmswithout the imidation during the removal of the organic solvents.Although applicable to other polyimide films, the films of the solublearomatic polyimides have low mechanical properties, in particular, tearstrength if the aromatic polyimide films have good mechanical propertiesas well as good heat resistance, they can be widely used as intermediatetransfer belts of machines such as electrophtographyic copying machines,flexible print circuits (FPC), or other structural members, electronicparts, optical parts, etc.

PROBLEMS TO BE SOLVED BY THE INVENTION

One object of the present invention is to provide a coating compositioncomprising a soluble aromatic polyimide which has good heat resistanceand mechanical strength, and the film formed from such a solublearomatic polyimide.

MEANS TO SOLVE THE PROBLEMS

To solve the above problem, firstly the present invention provides acoating composition comprising a polyimide soluble in organic solvents,wherein said polyimide is a compound of the formula (I):

wherein R's represent independently from one another a hydrogen atom, ahalogen atom, a C₁-C₁₀ alkyl group or a C₁-C₁₀ alkoxy group, a phenylgroup, or a phenyl group which is substituted with 1 to 4 halogen atomsand/or C₁-C₁₀alkyl groups;A is a C₆-C₂₀ tetravalent aromatic group or a group of the formula (II):

wherein B is a covalent bond, a >C(R²)₂ group, a carbonyl group, anoxygen atom, a sulfur atom, a >SO₂ group, a >Si(CH₃)₂ group, a>Si(C₂H₅)₂ group, a >NR³ group or a bifunctional ether group in which R²is a hydrogen atom or C(R⁴)₃ R³ is a hydrogen atom, a C₁-C₂₀ alkyl groupor a C₆-C₂₀ aryl group, and R⁴ is a hydrogen atom, a fluorine atom or achlorine atom; Ar is a monocyclic or polycyclic C₆-C₂₀, aromatic nucleusor a group of the formula (III):

wherein Ar and B are the same as defined above, and each p is an integerof 0 to 10; m is an integer of 1 to 10,and said polyimide has a weight average molecular weight of 150,000 to1,000,000 in terms of a polystyrene molecular weight measured with gelpermeation chromatography.

Secondly, the present invention provides a coating compositioncomprising a polyimide soluble in organic solvents, wherein saidpolyimide is a polycondensate polymer of3,3′,4,4′-biphenyltetracarboxylic dianhydride,

a diamine having an aromatic ether group,

a 9,9-bis (4-amylaryl) fluorene, znc optionally pyromellitic dianhydride(1,2,4,5-benzenetetracarboxylic dianhydride),

and said polyimide has a weight average molecular weight of 150,000 to1,000,000 in terms of a polystyrene molecular weight measured with gelpermeation chromatography.

Thirdly, the present invention provides a film formed from the abovecoating composition of the present invention.

EMBODIMENTS TO CARRY OUT THE INVENTION

Hereinafter, the coating composition of the present invention and a filmformed therefrom will be explained by making reference to preferredembodiments, but they do not limit the scope of the present invention.

The coating composition of the present invention is prepared bydissolving the soluble polyimide in an organic solvent. The polyimide isthe aromatic polyimide having a plurality of pendant fluorene structuralunits, as represented by the formula (I). The pendant fluorenestructural units allow the polyimide to be dissolved in the organicsolvent.

The chemical structure of the formula (I) will be explained in detail.

The four R groups included in one repeating unit are independent fromone another. That is, they may be the same or different. Each R is ahydrogen atom, a halogen atom, a C₁-C₁₀ alkyl group or a C₁-C₁₀ alkoxygroup a phenyl group, or a phenyl group which is substituted with 1 to 4halogen atoms and/or C₁-C₁₀ alkyl groups.

m is an integer of 1 to 10.

Raw materials corresponding to the polyamide having such a chemicalstructure may be commercially available or can be easily synthesized.

A is a C₁-C₂₀ tetravalent aromatic group or a group of the above formula(II).

In the formula (II), B is a covalent bond, a >C(R²)₂ group, a carbonylgroup, an oxygen atom, a sulfur atom, a >SO₂ group, a >Si (CH₃)₂ group,a >Si(C₂H₅)₂ group, a >NR₃ group, or a bifunctional ether group (e.g. agroup derived from a dihydric phenol such as bisphenol A, from thephenolid hydroxyl groups of which hydrogen atoms are removed). In thesegroups, R² is a hydrogen atom or —C (R⁴)₃, R³ is a hydrogen atom, aC₁-C₂₀ alkyl group or a C₆-C₂₀ aryl group, and R⁴ is a hydrogen atom, afluorine atom or a chlorine atom.

Ar in the formula (I) is a monocyclic or polycyclic C₆C₂₀, aromaticnucleus or a group of the formula (III). Two suffixes p in the formula(III) are the same or different, and each p is an integer of 0 to 10.

The weight average molecular weight of the soluble aromatic polyimide ofthe formula (I) is from 150,000 to 1,000,000, preferably from 200,000 to600,000, in terms of a polystyrene molecular weight measured with gelpermeation chromatography (GPC). When, the molecular weight is less than150,000, the produced film has low mechanical strength. When themolecular weight exceeds 1,000,000, a solution having a suitableconcentration may not be prepared.

In the present invention, a preferable aromatic carboxylic anhydrideconstituting the soluble aromatic polyimide is3,3′,4,4′-biphenyltetracarboxylic dianhydride. Thus, the residue derivedfrom such a dianhydride preferably constitutes the group A, since such agroup can impart good elasticity to the film or sheet formed from thesoluble aromatic polyimide. 3,3′,4,4′-Biphenyltetracarboxylicdianhydride is commercially available under the trade name of s-BPDAfrom Ube Industries, Ltd.

Preferably, pyromellitic dianhydride is used as a further aromaticcarboxylic anhydride. The use of pyromellitic dianhydride decreases theelastic modulus of the film or sheet of the aromatic polyimide, and thusthe elastic modulus of the film or sheet can be adjusted in accordancewith the applications of the film or sheet.

The diamine which constitutes the soluble aromatic polyimide preferablyhas an aromatic ether functional group and in turn, such a functionalgroup constitutes (Ar)_(m), from the viewpoint of the flexibility of thefilm or sheet formed from the soluble aromatic polyimide. In particular,when the diamine is diaminodiphenyl ether (ODA) or1,3-bis(4-aminophenoxy)benzene, it can impart flexibility to the film orsheet without deteriorating the durability of the polyimide.

The fluorene is usually 9,9-bis(4-aminoaryl)fluorene, preferably9,9-bis(4-aminophenyl)fluorene (BAF) or9,9bis(4-amiono-3-methylphenyl)fluorene (OTBAF), since they can berelatively easily available.

The soluble aromatic polyimide may be prepared as follows:

The acid anhydride, the diamine and the fluorene derivative are mixed ina specific molar ratio. Then, the mixture is stirred at room temperaturefor a specific time to form a polyamic acid. Next, the polyamic acid isimidated with acetic anhydride and preferably absolute (dry) pyridine.More in detail, acetic anhydride and anhydrous pyridine are added to thepolyamic acid to form a solution. Then, the solution is heated at 120°to 140° C. while stirring to obtain the aromatic polyimide. Then, thearomatic polyimide is cooled to about 25° C. followed by purificationwith tetrahydrofuran (THF) and water.

When THF is used, it is possible to isolate high purity aromaticpolyimide having a high molecular weight and a saturation solubility inTBF of 50 g/liter or less, preferably 10 g/liter or less, from theunreacted monomers, low molecular weight aromatic polyimide and animidization catalyst used.

Subsequently, the purified aromatic polyimide is dried to obtain thepowder product.

The polyimide, which is produced as described above, has good heatresistance due to its aromatic nature. In addition, the aromaticpolyimide is hardly decomposed since the imidization is completed. As aresult, it is not necessary to store or treat the aromatic polyimideunder dry conditions.

In general, the aromatic polyimide is dissolved in the organic solventto prepare the coating composition. The inorganic solvent is not limitedin the present invention. Preferred examples of the organic solventinclude organic polar solvents such as N-methylpyrrolidone (NMP),1,3-dimethyl-2-imidazolidinone, dimethylsulfoxide,N,N,N′,N′,N″,N″-hexamethylphosphorylamide.

When A is the group of the formula (II) in which B is a bifunctionalether group, the aromatic polyimide can be dissolved in an amide solvent(e.g. N,N-dimethylformamide (DMF), N,N-dimethylacetoamide, etc.) or anether solvent (e.g. tetrahydrofiran (THF), dioxane, cyclopentanone,cyclohexanone, etc.).

The concentration of the aromatic polyimide in the coating compositionmay be suitable adjusted according to the thickness of the film or sheetformed. Typically, 8 to 20 parts by weight of the aromatic polyimide isdissolved in 92 to 80 parts by weight of the organic solvent. When theamount of the aromatic polyimide dissolved is less than 8 parts byweight, it is necessary to thickly apply the coating composition to formthe film or sheet having a desired thickness, and thus it takes a longtime to dry the applied composition. on the other hand, it is verydifficult to dissolve more than 20 parts by weight of the aromaticpolyimide.

The coating composition can be applied on a substrate with a coatingmeans such as a knife coater at a desired thickness, and then theorganic solvent is removed by heating and drying to form a heatresistant film. In the formation of the film, the heating and dryingtime of the applied solution can be shortened when the soluble aromaticpolyimide according to the present invention is used, since theimidation of the aromatic polyimide is completed as explained above, andthus the undesirable by-products (for example, water), which should beremoved together with the organic solvent, are not generated. Inparticular, in the case of the coating composition comprising thearomatic polyimide and the organic polar solvent, the heating and dryingof the applied composition can be completed in a relatively short time,and thus such a composition is suitable for substrates having low heatresistance (for example, electronic parts such as FPC, or opticalparts).

According to the present invention, the film of the polyimide has goodmechanical properties such as tear strength, elastic modulus, etc. Inparticular, the tear strength is rather high. When the film thickness isin the range between 40 and 150 μm, a tear strength of 160 to 490 N/mm,preferably 185 to 490 N/mm, more preferably 190 to 490 N/mm is achieved.Thus, the film can be advantageously used as, for example, a conveyingbelt.

When conductive fillers such as metal oxides, carbon black, etc. areuniformly dispersed in the coating composition of the present invention,a conductive film having a volume resistivity of 10⁸ to 10¹² Ωcm can beobtained.

When such a film is formed in the form of a seamless product (that is, aloop form), it can be used as an intermediate transfer belt or a fixingbelt used in the electrophtographyic copying machines. Particularly insuch applications, the thickness of the film is usually from 50 to 150μm, preferably from 70 to 130 μm. When the film thickness is less thanthe lower limit, the film has insufficient strength. When the filmthickness exceeds the upper limit, the film has low flexibility. Thefilm having such a thin thickness can produce a belt having sufficientstrength.

The present invention has been explained by making reference to thepreferred embodiments, but the invention is not limited to thoseembodiments.

For example, the acid anhydride as the raw material of the polyimide isnot limited to 3,3′,4,4′-biphenyltetracarboxylic dianhydride, and anyother carboxylic anhydrides may be used insofar as they are aromaticones. Examples of other acid anhydrides include pyromellitic anhydride,bis(phthalic anhydride)sulfone, bis(phthalic anhydride) ether,2,2-bis(phenoxyphthalic anhydride)propane,1,1,1,3,3,3-hexafluoropropane-2,2bis(phthalic anhydride),2,3,3′,4′-biphenylte tracarboxylic dianhydride,benzophenonetetracarbocylic dianhydride, etc.

Also, the diamine is not limited to the diamine having the aromaticether group, and any other diamines may be used insofar as they arearomatic ones. Examples of the other diamines includebis(4-aminophenyl)methane, 2,2-bis(4-aminophenyl)propane,phenylenediamine, bis(4-aminophenyl)sulfone, etc.

Furthermore, the fluorene is not limited to9,9-bis(4-aminoaryl)fluorene, and any other fluorene derivatives may beused insofar as they are bifunctional ones. Examples of the otherfluorene derivatives include 9,9-bis(4-aminophenoxyphenyl)fluorene, etc.

The present invention will be illustrated by the following Examples,which do not limit the scope of the invention in any way.

EXAMPLE 1

Synthesis of Aromatic Polyimide

In a vessel placed in a nitrogen atmosphere, 34.85 g of9,9-bis(4-aminophenyl)fluorene (BAF), 20.02 g of 3,4′diaminodiphenylether (3,4′-ODA) (available from WAKAYAMA SEIKA KABUSHIKIKAISHA and58.83 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA)(available from Ube Industries, Ltd.) were mixed to prepare a solution,which was stirred at about 25° C. for 21 hours. Then, 81.3 g of aceticanhydride and 63.3 g of absolute pyridine were dropwise added to thevessel. Thereafter, the vessel was heated on an oil bath at 120 to 140°C. for 2 hours while stirring the solution.

Subsequently, the solution was cooled to about 25° C., and dropwiseadded to 3 liters of THF to obtain a precipitate. The precipitate wasonce isolated from THF by decantation, washed with water, immersed in1.5 liters of TEY for 18 hours, and again isolated from THF bydecantation.

The isolated precipitate was heated and dried in an oven at about 100°C. for 18 hours to obtain a powdery aromatic polyimide.

The weight average molecular weight of the obtained aromatic polyamidewas measured with gel permeation chromatography, and was about 290,000in terms of the polystyrene molecular weight.

Production of Aromatic Polyimide Film

Ten (10) grams of the powdery aromatic polyimide obtained in the abovestep was dissolved in 90 g of N-methylpyrrolidone (NMP) to obtain acoating composition containing 10% by weight of the aromatic polyimide.The coating composition was applied on a substrate consisting of a filmmanufactured by DuPont (CAPTON®) with a knife coater having a gap lengthof 800 μm. Then, the substrate carrying the applied coating compositionwas placed in an oven and heated and dried at about 100° C. for 18 hoursto obtain an aromatic polyimide film having a thickness of about 75 μm.Then, the substrate and the polyimide film carried thereon were heatedin an oven at about 250° C. for 30 minutes to remove substantially allthe solvent.

Evaluation of Aromatic Polyimide Film

The aromatic polyimide film was peeled off from the substrate and itstear strength was measured according to JIS K 7128-3.

The tear strength of the film of this Example was 265 N/mm.

EXAMPLE 2

An aromatic polyimide was synthesized, and a film was produced therefromand evaluated in the same manner as in Example 1 except that 52.26 g ofBAS, 10.01 g of 3,4′-ODA and 58.84 g of s-BPDA were used. The polyimidehad a weight average molecular weight was about 250,000 in terms of apolystyrene molecular weight measured with gel permeationchromatography, and the film had a tear strength of 167 N/mm.

An aromatic polyimide was synthesized in the same manner as in Example 1except that 1306.5 g of BAF, 365.5 g of TPE-R and 1471.1 g of s-BPDAwere used. The polyimide had a weight average molecular weight was about260,000 in terms of a polystyrene molecular weight measured by gelpermeation chromatography. A film was produced from this polyimide andevaluated. It had a tear strength of 216 N/mm.

EXAMPLE 4

The powdery aromatic polyimide prepared in Example 3 (50 g) wasdissolved in 450 g of NMP to obtain a coating composition containing 10%by weight of the aromatic polyimide. Then, the coating composition wasuniformly applied with a knife coater having a gap length of 900 μm onan inner wall surface of a hollow cylinder of stainless steel having aninner diameter of 438 mm and a width of 50 mm. Then, the cylindercarrying the coating composition applied was dried with a hot air usinga drier at about 80° C. for 8 hours to obtain a seamless film (or anendless loop film).

Then, the seamless film was removed from the hollow cylinder and heatedat about 250° C. for 30 minutes to remove substantially all the solvent.The heated seamless film had a thickness of about 75 μm. The tearstrength of the seamless film was 235 N/mm when measured according toJIS F 7128-3.

EXAMPLE 5

A heated seamless film was produced in the same manner as in Example 4except that a coating composition was prepared by dissolving anddispersing 30 g of the powdery polyamide and conductive titanium oxide(FT2000 manufactured by ISHIARA SANGYO KAISHA, LTD.) in 270 g of NMP.The tear strength of the seamless film was 167 N/mm when measuredaccording to JIS K 7128-3.

Comparative Example 1

A film was prepared and evaluated in the same manner as in Example 1except that a solution of a polyamide (RIACOAT SN 20 (trade name)manufactured by SHIN-NIPPON RIKA KABUSHIKIKAISHA, L weight averagemolecular weight of about 130,000 in terms of a polystyrene molecularweight measured by gel permeation chromatography) The tear strength ofthe film was 147 N/mm.

Effects of the Invention

The coating composition of the present invention can be dried in arelatively short time, and molded products such as a film or a sheetproduced therefrom have good heat resistance and mechanical strength, inparticular, tear strength.

1. A seamless conveying belt comprising a polyimide soluble in organicsolvents, wherein said polyimide is a compound of the formula (I):

wherein R's represent independently from one another a hydrogen atom, ahalogen atom, a C₁-C₁₀alkyl group or a C₁-C₁₀ alkoxy group, a phenylgroup, or a phenyl group which is substituted with 1 to 4 halogen atomsand/or C₁-C₁₀ alkyl groups; A is a C₆-C₂₀ tetravalent aromatic group ora group of the formula (II):

wherein B is a covalent bond, a >C(R²)₂ group, a carbonyl group, anoxygen atom, a sulfer atom, a >SO₂ group, a >Si(CH₃)₂ group, a>Si(C₂H₅)₂ group, a >NR³ group or a bifunctional ether group in which R²is a hydrogen atom or C(R⁴)₃, R³ is a hydrogen atom, a C₁-C₂₀ alkylgroup or a C₆-C₂₀ aryl group, and R⁴ is a hydrogen atom, a flourine atomor a chlorine atom; Ar is a monocyclic or polycyclic C₆-C₂₀ aromaticnucleus or a group of the formula (III):

wherein Ar and B are the same as defined above, and each p is an integerof 0 to 10; m is an integer of 1 to 10, and said polyimide has a weightaverage molecular weight of 150,000 to 1,000,000 in terms of apolystyrene molecular weight measured with gel permeationchromatography.
 2. A seamless conveying belt comprising a polyimidesoluble in organic solvents, wherein said polyimide is a polycondensatepolymer of 3,3′,4,4′-biphenyltetracarboxylic dianhydride, a diaminehaving an aromatic ether group, and 9,9-bis(4-amylaryl)fluorene, andsaid polyimide has a weight average molecular weight of 150,000 to1,000,000 in terms of a polystyrene molecular weight measured with gelpermation chromatography.
 3. The seamless conveying belt according toclaim 1, wherein a saturation solubility of said polyimide intetrahydrofuran is 50 g/liter or less.
 4. The seamless conveying beltaccording to claim 2, wherein a saturation solubility of said polyimidein tetrahydrofuran is 50 g/liter or less.
 5. The seamless conveying beltaccording to claim 2, wherein said polyimide further comprisespyromellitic dianhydride as a monomer.
 6. The seamless conveying beltaccording to claim 2, wherein said diamine is at least one diamineselected from the group consisting of diaminodiphenyl ether and1,3-bis(4-aminophenoxy)benzene.
 7. The seamless conveying belt accordingto claim 2, wherein said 9,9-bis(4-aminoaryl)fluorene is at least onefluorene derivative selected from the group consisting of9,9-bis(4-aminophenyl)fluorene and9,9-bis(4-amiono-3methylphenyl)fluorene.
 8. The seamless conveying beltaccording to claim 1 additionally comprising conductive fillers.
 9. Theseamless conveying belt according to claim 2 additionally comprisingconductive fillers.
 10. The seamless conveying belt according to claim 9having a volume resistivity of 10⁸ to 10¹² ohm·cm.
 11. The seamlessconveying belt according to claim 9 having a volume resistivity of 10⁸to 10¹² ohm·cm.
 12. The seamless conveying belt according to claim 1,which has a tear strength of 160 to 490 N/mm according to JIS K 7128-3.13. The seamless conveying belt according to claim 2, which has a tearstrength of 160 to 490 N/mm according to JIS K 7128-3.